An electronic key system for a vehicle that controls a door locking operation and an engine starting operation based on a result of wireless communication between an in-vehicle unit and a portable device, such as an electronic key, has been known. Such an electronic key system is described in patent citations JP 3659583 B2, JP 2008-240315 and JP 4366376 B2.
In such an electronic key system, a request signal (primary signal) is transmitted from a transmitting antenna disposed in a vehicle to a communication area provided inside or outside of a passenger compartment. When the portable device is inside of the communication area, the portable device transmits a response signal (secondary signal) to the in-vehicle unit in response to the request signal. When the response signal satisfies a predetermined condition, the in-vehicle unit performs a predetermined operation, such as a door unlocking operation.
As described in the patent citation 1, transmitting antennas are disposed at different positions of a vehicle, such as doors, a center console and rear seats. Each of the transmitting antennas provides a communication area such that the communication areas do not overlap with each other. In particular, a communication area provided by an interior transmitting antenna and a communication area provided by an exterior transmitting antenna are adjusted not to overlap with each other.
With regard to systems described in the patent citations 2 and 3, a portable device has a function of measuring an intensity of a request signal, such as a received signal strength indication (RSSI) value and a radio field intensity, transmitted from a vehicle. Further, with regard to the system of the patent citation 2, the portable device transmits a response signal including the RSSI value measured to an in-vehicle unit, and the in-vehicle unit restricts a relay attack based on the RSSI value of the response signal.
FIG. 12 is a diagram of a receiving circuit 900 employed in such a portable device for receiving a request signal. Also, FIG. 5A illustrates a change of a demodulated waveform of the request signal at respective portions of the receiving circuit 900 when the portable device is distant from a transmitting antenna, and FIG. 5B illustrates a change of the demodulated waveform of the request signal at the respective portions of the receiving circuit 900 when the portable device is near the transmitting antenna.
The receiving circuit 900 includes a receiving part 910, a demodulating part 920 and a waveform shaping part 930. The receiving part 910 includes an antenna coil 911 and a capacitor 912 to form a resonance circuit. The demodulating part 920 performs an envelope demodulation of a waveform received at the receiving part 910. The waveform shaping part 930 shapes the envelope-demodulated waveform into a binary signal. The waveform shaping part 930 includes a threshold setting circuit 931. The waveform shaping part 930 outputs a signal indicting a high level or a low level, that is, the binary signal, by comparing a value of the envelope-demodulated waveform to a threshold. Also, the threshold setting circuit 931 serves as a low-pass filter that moderates the envelope-demodulated waveform. The threshold is provided by an output of the low-pass filter.
In FIGS. 5A and 5B, (1) illustrates a base-band waveform of the request signal, and (2) illustrates a waveform at an output point 913 of the receiving part 910. Also, (3) illustrates a waveform at an output point 921 of the demodulating part 920, and (4) illustrates a waveform (demodulation waveform) at an output point 933 of the waveform shaping part 930. Further, dashed lines 941, 942 of (3) of FIGS. 5A and 5B illustrate a change of the threshold set in the threshold setting circuit 931, that is, a value at an output point 932.
As shown in (2) of FIGS. 5A and 5B, rising and falling of the waveform received by the receiving part 910 is slowed relative to the base-band waveform of the request signal shown in (1) of FIGS. 5A and 5B due to an effect of antenna Q value of the antenna coil 911. An intensity of the electric field generated from the transmitting antenna changes in inverse proportion to the cube of a distance. Therefore, when the portable device is near the transmitting antenna, the portable device necessarily receives a very strong radio wave, as shown in (2) of FIG. 5B.
In this case, the threshold cannot follow a change of the envelope-demodulated waveform in a section due to an increase in an amplitude of the envelope-demodulated waveform and an influence of a time constant of the threshold setting circuit 931, as shown in (3) of FIG. 5B. As a result, the demodulation waveform misses a bit, as shown in (4) of FIG. 5B.
Accordingly, in a system where the threshold is set based on the waveform slowed due to the antenna Q value of the portable device and/or the envelope-demodulated waveform, when the portable device is located near the transmitting antenna, it is difficult to properly receive the request signal in the portable device.
Patent Citation 1: Japanese Patent No. 3659583 B2
Patent Citation 2: Japanese Patent Application Publication No. 2008-240315
Patent Citation 3: Japanese Patent No. 4366376 B2